This invention relates generally to micro-image techniques for recording and reproducing intelligence, and more particularly to a microfilm system in which specially prepared, interlaced microimages appearing on a multiple-image microrecord may be read back by a highly compact optical reader.
In order to conserve storage space and afford a ready means for retrieving information, it is now common practice to photograph documents, records, books, and various forms of technical data in a sharply reduced scale on microfilm. Conventionally, a microscale image of each book page or document is recorded on a separate frame on a microfilm roll. To view the recorded data, the roll is inserted in a reader or optical projector and the film is advanced therein to place a selected frame into the optical gate whereby an enlarged and readable image is cast on a screen.
In recent years, a new technique has been developed to place a group of microfilm images on a single sheet rather than along a film roll. In such sheets, which are known commercially as "microfiches," the microscale images are generally arranged serially along parallel rows, whereby the microimages are distributed in a grid formation on the sheet. A microfiche is useful where one wishes to incorporate on a single reproducible sheet, interrelated documents or other information pertaining to a particular subject matter. Thus, with a microfiche, one may record all of the pages of a sizeable book on a single sheet.
An original microfiche is known as a master. The principle advantage of a microfiche master, apart from the fact that it provides a consolidated record, is that it is readily reproducible to form additional or reference copies. The most effective way of deriving transparent reference copies from a microfiche master is by the contact printing process, the microfiche master serving as the negative.
By using modern microfiche techniques, one is able, from a single microfiche master containing all of the pages of a book, to produce thousands of low-cost reference copies. It becomes possible, therefore, at a modest cost, to supply a student with a small file containing, say, five hundred microfiches, each being a complete record of a reference book. In this way, an engineering student may be provided with a complete library of 500 engineering texts at a very small fraction of the cost of these books.
In order, however, to make use of the microfiche, the student requires an optical reader. This reader must include means to shift the microfiche in the X and Y directions, in order to align a particular frame with the optical system serving to project an enlarged image of the selected frame or page on the screen.
Existing forms of microfilm and microfiche readers, even those of very simple and relatively compact design, have a certain minimum volume which precludes their use in the same manner as a book. A typical book is about 8 inches wide, 10 inches long and 11/2 inches in depth. A book of this or somewhat larger size may be carried about and read without difficulty, for the book dimensions lend themselves to personal handling.
But though a microfilm roll, a microfilm cartridge or a microfiche makes it possible to highly compress the information contained on hundreds or thousands of pages, the conventional reader adapted to reproduce the images carried on the film strip or microfiche has a substantial volume, so that even the smallest commercially available reader cannot be carried about or handled in the manner of a book.
In order to understand why existing reader designs dictate a relatively large minimum volume that precludes dimensions comparable to those of a book, we shall consider by way of example a microfiche containing 2160 frames, each frame being a microimage of a book page, whose dimensions at 81/2 .times. 11 inches, with several thousand characters appearing on each page. For proper read-out without eyestrain, a frame magnification of about 60 times is required. Obviously, the larger the magnification, the greater the acuity in viewing or reading of the micro-recorded material.
However, practical considerations impose strict limits on the degree of magnification which is feasible. Let us assume that the projection system must be adapted to blow-up the microimage from a size of 0.168 .times. 0.130 inches, to a size equal to or larger than the original of 81/2 .times. 11 inches. Since lens systems are generally round, the lens must be capable of accommodating the larger diagonal of the 81/2 .times. 11 inch page without distortion.
This projector requires a lens of large aperture (small f-number) and large field acceptance angle. In order to present enlarged letters in acceptable form with only a negligible degree of distortion at the corners of the page, the lens must be highly corrected. Such lenses are expensive and run as high as $150 or more.
The f-number has to be small, if one considers that all light appearing on the screen must pass through the very small microimage frame. The smaller the f-number, the less wasteful is the lens in optical terms, but the more expensive it becomes. A severe limit on the amount of permissible light is set by the fact that the microfiche may be damaged by absorbed light flux. If the light intensity is increased to improve the readability of the image, the resultant increase in absorbed light flux may be destructive to the film.
A typical commercial reader lens suitable for existing microfiche images may have as many as 12 elements and a length of 3.7 inches, with a focal length of 6.2 mm and a diameter of about 1.2 inches. A lens of this type is suitable for only a small field of about 11.degree. and requires a projection distance of at least 40 inches.
It is for the foregoing reasons that conventional microfilm or microfiche readers must have a minimum light path of 12 to 40 inches for acceptable performance. Such readers, therefore, usually make use of beam-folding arrangements in order to create a relatively compact reader box whose dimensions make it feasible to place the reader on a table or pedestal. But with the existing microfilm and microfiche techniques, however ingeniously the optical reader is designed, one cannot, because of minimum light path requirements, reduce the reader dimensions to a point where the overall dimensions are comparable to those of a book, that is a reader in shallow box form whose length and width, determined by screen size, more or less correspond to those of a book page, and with a box depth no greater than about two inches.